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Bad performance of a typical 1 A/12 V power adapter

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E-design

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I was testing some low-cost eBay switching power adapters and saw all had sloppy performance on step load testing.

I opened it up and decided to test the gain/phase response at 500 mA.

Below is the plots from the FRA used. Looking at the first plot it can be seen why the response is so bad.

On further investigation, it appears that the manufacturer used such a massive value of roll-off capacitor without any testing or verification. On the application note of the flyback chip used in this, the capacitor (C4) value is for the designer to select and optimize. Since no value was indicated, someone thought that a 100 nF seemed like a good idea. The bigger the better, hey?

Reducing this to 10 nF made a good improvement in performance.
 

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What is the phrase? One gets what one pays for?

I've seen worse (and dangerous) mistakes, like minimal separation and insulation between mains and the output.
 

This is supposed to be one of the better higher-performance low-noise power supplies. Most other really cheap ones do not employ a switching regulator chip but use a self-oscillating transistor configuration without any short circuit protection.
 

I'm fairly certain the makers never heard of a frequency response analyzer.
 

This is supposed to be one of the better higher-performance low-noise power supplies. Most other really cheap ones do not employ a switching regulator chip but use a self-oscillating transistor configuration without any short circuit protection.

You are correct. This one at least has certain degree of sophistication, although the design engineer wasn't well acquainted with feedback theory.
 

It appears that it was more a case of copying the application circuit and guessing the unmarked capacitor value.
 

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